The neonatal hyperbilirubinemia study has been designed to assess the relationship of intermediate levels of serum bilirubin on the subsequent neurological and mental development of NINCDS Collaborative Perinata Project children. There has been increasing concern that neonatal serum bilirubin levels between 10-20 mg% may be damaging to the central nervous system, not in the classical sense of 'kernicterus' associated with levels above 20 mg%, but rather damaging in more subtle yet clinically significant ways. Neonates have been studied in five birthweight-gestational age categories, by three socioeconomic classes, for a variety of outcome measures, including mental and motor assessment at age 8 months, and a spectrum of neurological findings at age one year which included motor performance, reflexes, tone, abnormal movements, eye findings and the overall neurological classification of normal, suspect or abnormal. The analysis of Phase I of this study has been published. The analysis of Phases II and III, which include data obtained at ages four and seven years, has been completed. Findings were generally negative.